fix a bug in RemoveAttribute that accidentally uses GetVariableMap
fix a bug in RemoveAttribute that accidentally uses GetVariableMap

That originated as copy&paste error, I presume.

ornladios/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2